Curved cuts within a transparent material are generated, in particular, in laser-surgical methods, especially in opththalmic surgery. This involves focusing treatment laser radiation within the tissue, i.e. beneath the tissue surface, so as to form optical breakthroughs in the tissue.
In the tissue, several processes initiated by the laser radiation occur in a time sequence. If the power density of the radiation exceeds a threshold value, an optical breakthrough will result, generating a plasma bubble in the material. After the optical breakthrough has formed, said plasma bubble grows due to expanding gases. If the optical breakthrough is not maintained, the gas generated in the plasma bubble is absorbed by the surrounding material, and the bubble disappears again. However, this process takes very much longer than the forming of the bubble itself. If a plasma is generated at a material boundary, which may quite well be located within a material structure as well, material will be removed from said boundary. This is then referred to as photo ablation. In connection with a plasma bubble which separates material layers that were previously connected, one usually speaks of photo disruption. For the sake of simplicity, all such processes are summarized here by the term optical breakthrough, i.e. said term includes not only the actual optical breakthrough, but also the effects resulting therefrom in the material.
For a high accuracy of a laser surgery method, it is indispensable to guarantee high localization of the effect of the laser beams and to avoid collateral damage to adjacent tissue as far as possible. It is, therefore, common in the prior art to apply the laser radiation in a pulsed form, so that the threshold value for the power density of the laser radiation required to cause an optical breakthrough is exceeded only during the individual pulses. In this regard, U.S. Pat. No. 5,984,916 clearly shows that the spatial extension of the optical breakthrough (in this case, of the generated interaction) strongly depends on the pulse duration. Therefore, precise focusing of the laser beam in combination with very short pulses allows placement of the optical breakthrough in a material with great accuracy.
The use of pulsed laser radiation has recently become established practice particularly for laser-surgical correction of visual deficiencies in ophthalmology. Visual deficiencies of the eye often result from the fact that the refractive properties of the cornea and of the lens do not cause optimal focusing on the retina.
U.S. Pat. No. 5,984,916, mentioned above, as well as U.S. Pat. No. 6,110,166 describe methods of the above-mentioned type for producing cuts by means of suitable generation of optical breakthroughs, so that, ultimately, the refractive properties of the cornea are selectively influenced. A multitude of optical breakthroughs are joined such that a lens-shaped partial volume is isolated within the cornea. The lens-shaped partial volume which is separated from the remaining corneal tissue is then removed from the cornea through a laterally opening cut. The shape of the partial volume is selected such that, after removal, the shape and the refractive properties of the cornea are thus modified so as to have the desired correction of the visual deficiency. The cuts required here are curved, which makes a three-dimensional adjustment of the focus necessary. Therefore, a two-dimensional deflection of the laser radiation is combined with simultaneous adjustment of the focus in a third spatial dimension.
The two-dimensional deflection of the laser radiation and the focus adjustment are both important for the accuracy with which the cut can be produced. At the same time, the speed of adjustment, which is achievable thereby, has an effect on the speed at which the required cut can be produced. Generating the cuts quickly is desirable not only for convenience or in order to save time; bearing in mind that movements of the eye inevitably occur during ophthalmological operations, quick generation of cuts additionally contributes to the optical quality of the result thus achieved and avoids the requirement to track eye movements.
Therefore, it is an object of the invention to improve a method and an apparatus of the above-mentioned type such that the time required to generate a cut is as short as possible.